Before you can understand Port Forwarding there are a couple of concepts you need to know.

What an IP address is.
 
Basiclly an IP Address is a name for a computer,  every computer that wants to send or receive data on the internet needs to have an IP address. Because an IP address acts as an identification number, no two computers on the internet can have the same IP address.  The IP address is a number that is used to identify a device, a set of 4 three digit numbers divided by periods. The numbers of an IP address range from 0 to 255.


What are Ports?

A port number represents an endpoint or "channel" for network communications.  Port numbers allow different applications on the same computer to utilize network resources without interfering with each other.   Every IP address is divided up into many ports.  When one computer sends data to another computer, it sends it from a port on an IP address to a port on an IP address.  Port numbers can theoretically range from 0 to 65535. Most popular network applications, though, use port numbers at the low end of the range (such as 80 for HTTP).  Any one port can only be used by one program at a time.
For example,  ports make it possible for you to check your email and browse the web at the same time. This is possible because browsing the web uses port 80, and getting your email uses port 110.



What are TCP and UDP ports for?

TCP stands for Transmission Control Protocol.

The way TCP works is the computer sending the data connects directly to the computer it is sending the data to, and stays connected for the duration of the transfer. The two computers can guarantee that the data has arrived safely and correctly, and then they end the connection.
For example,  when you pick up the phone and call a friend, you have a conversation and when it is over, you both hang up.

UDP stands for User Datagram Protocol.

The way UDP works is the computer sending the data packs the information into a nice little package and releases it into the network with the hopes that it will get to the right place. What this means is that UDP does not connect directly to the receiving computer like TCP does.  Rather it sends the data out and relies on the devices in between the sending computer and the receiving computer to get the data where it is supposed to go properly.
This method of transmission does not provide any guarantee that the data you send will ever reach its destination.  
For example, you place your mail in the mailbox and hope the it gets it to the proper location. Most of the time it does, but sometimes it gets lost along the way.

UDP is part of the TCP/IP suite used by applications to transfer datagrams. It is also the part of TCP/IP responsible for port addresses.
When information is sent over the internet to your computer it accepts that information by using TCP or UDP ports.


What does NAT mean?

NAT is an acronym for Network Address Translation.  Using NAT your router directs the data, to multiple internal private IP addresses.  NAT is simply the technical term used for the process of determining the destination of data packets sent to your network from the internet.
There is a problem with it though. When a computer on the internet wants to retrieve data from a computer on your network, your router can't figure out which internal computer to get the data from.
This is where port forwarding comes in. Port forwarding tells your router which internal computer to get the data from when it sees data on certain ports.




Here the external IP address is coming from the Wide Area Network (WAN) broken into multiple internal IP addresses.  The router has two IP addresses. It has the external IP address, and an internal IP address which acts as the gateway for every computer in your Local Area Network (LAN). The other two Internal IP adresses are the IP addresses of the computers (a seperate addresses for each one).

Computers on the LAN can only "see" internal IP addresses. So computers on the internal network can not send data directly to a computer outside in the WAN.   When a computer on the network wants to send data to a computer outside of the network, it sends the data to the gateway.  The router then takes this data and sends it out to the computer on the internet from the external IP address.
The same thing is true of computers on the internet.  A computer outside of the LAN can not "see" a computer inside of the it. They can only "see" and send data to the external IP address of the router. The router must then decide what to do with this data.
This is where the NAT (Network Address Translation) comes into play, and with most programs it takes care of directing the data correctly for us.  However, there are a few programs that NAT was not designed to work with, those are the programs we need to set up port forwarding for, for example WinMX.


What is Port Forwarding?

Hopefully, you understand the general concepts and terminology of a network now, so understanding the port forwarding itself is easy.
When a computer on the internet sends data to the external IP address of the router, the router needs to know what to do with the data. Port Forwarding simply tells the router which computer on the local area network to send the data to. When you have port forwarding rules set up, your router takes the data off of the external IP address and port number and sends that data to an internal IP address and port number.  

A port can only be used by one program at a time. Port Forwarding rules set up for port 6699 will only work for port 6699, for each internal IP address.  You've only got one external IP address on your router. When one of your computers is using port 6699, it is using port 6699 on it's internal IP address.  If you have set up a port forwarding rule for that computer and port 6699, the external IP address's port 6699 is also in use. This means that you can only use port 6699 on one computer on the network at a time. Using port 6699 on two computers at the same time would violate the one program rule.  
Most routers require you to specify an internal IP address to forward ports to, for just this reason.  Port Forwarding rules will only work for one computer at a time, so if your planning on using more than one of your computers for the same program you need to use different ports for each internal IP.


A few other terms to know before you begin Port Forwarding your Router

DNS- Domain Name Service. The dns server basically translates a URL into the IP address of the server hosting the URL. If you type PortForward.com in your web browser, your computer needs to know what IP address corresponds to portforward.com. Your computer contacts the dns server, and "tells" it PortForward.com. The dns server then "tells" your computer 216.104.71.5. If you ping PortForward.com, the first line should be something that includes PortForward.com [216.104.71.5]. This basically tells you if the dns server was able to find the IP address for the domain name you submitted.

Domain Name - A domain name is what you usually type into the web browser when you want to view a page without the www part. Port Forward's  domain name is Portforward.com. Others you may be familiar with are google.com, yahoo.com...

Dynamic IP Address - The dynamic IP address is not really a type of IP address. The dynamic only relates to how an IP address is assigned. Your router hands out these IP addresses to computers every time they are rebooted. This means that every time your computer is rebooted, its IP address can change. You should have a static, not dynamic IP address if you are trying to do port forwarding. In a port forwarding configuration you must specify an IP address to forward the ports to. Let's say you do that, and then your IP address changes. The ports are now forwarded to the wrong IP address, so your port forwarding configuration stopped working.

Gateway - Usually this is your router. When you talk about your gateway, generally you are talking about the IP address your router uses. When you specify gateway in your TCP/IP properties you are entering the IP address of your router. If you enter your gateway into the browser bar, you get the web interface of your router. That's assuming your router has a web interface.

NIC - Network Interface Card. This is a card in your computer that allows you to connect to routers/hubs/modems via ethernet. It's the thing your network cable plugs into.

Port Forwarding - Allowing your router to direct ports to a private IP address. The data on these ports is always incoming data, and always originates on an external public IP address.

Port Triggering - Allowing your router to dynamically open up ports, when it sees data on other ports. Port triggering allows you to do port forwarding, but only have the ports open when you are running the application. So you open up a program. That program then sends data out on outgoing ports called trigger ports. Your router sees data leaving your network on these ports. The router then allows incoming data on the ports specified in your port triggering configuration. When the router no longer sees data going out on the trigger ports, it turns off access to the incoming ports.

Private(internal)IP Address - If you are using NAT, every computer on your network has one of these IP addresses. These IP addresses are provided by your router. Your entire network can see this IP address. Computers on the outside of your network can not, because your router "hides" private IP addresses.

Public(external) IP Address - Every computer/server that has a public IP address is directly connected to the internet. Usually DLS/cable routers are assigned a public IP address by the ISP they are connecting to.

Static IP Address - Static doesn't really refer to the type of IP address. It refers to how your computer obtains that IP address. Static IP addresses are hard coded into your computer by you, by hand. If you assign a computer on your network a static IP address, all the other computers on the network must have a static IP address. For more information on assigning static IP addresses visit our Static IP's page.

Subnet Mask - The subnet mask is part of your IP address configuration. The subnet tells your computer which group of IP addresses you belong to. For more information on subnet masks visit our Subnetting page.

TCP/IP - This is the protocol your computer uses to communicate with other computers on the internet, and within your network. In windows this portion of your network properties, allows you to define an IP address, subnet mask, gateway, and dns servers.


As mentioned above, there is a wide variety of routers.  There are a few routers and modems that also contain a built in firewall.  In order to foward ports, it will be necessary to have information about the brand and model number of your device.  Once you have this you can go to Port Foward.com.  This is a great site with detailed information and instructions on nearly every router, dsl/cable modem.

Great thanks go to Port Forward.com for thier great work.